General principles of structure and classification
The main morphological criterion of any stratified epithelial tissue is that exclusively the lower row of cells is connected to the basement membrane. All other layers lie above it and lack this contact.
The division of stratified epithelia into subgroups is based on two key features:
- Presence or absence of the keratinization process (keratinization).
- The ability of the entire cell layer to change its thickness (e.g., during organ distension).
The life cycle of the tissue depends on stem cells, which are always located in the basal layer. Following mitotic division, some cells enter the differentiation process. As they age, they are gradually displaced upward. Upon reaching the apical (superficial) zone, old cells desquamate. Although this renewal mechanism operates regularly, it proceeds slower than in simple epithelia.
Stratified squamous non-keratinizing epithelium
This tissue exhibits pronounced stratification. Despite the total number of cell rows potentially exceeding ten, histologically the layer is divided into three main zones:
- Basal layer. Located directly on the basement membrane. Includes stem and differentiating cells. Nuclei are oval-shaped and elongated perpendicularly to the membrane (oriented vertically).
- Prickle cell (intermediate) layer. Is stratified. Cells acquire an irregular polygonal shape, and their nuclei become rounded. Cells form short processes (so-called "prickles") that firmly connect with the processes of neighbors via intercellular contacts—desmosomes.
- Squamous cell (superficial) layer. Is also stratified. Cells here are flattened, and their narrow, rod-shaped nuclei are elongated parallel to the surface of the layer. Over time, these cells desquamate.
Note: It is precisely the shape and horizontal orientation of the nuclei in the superficial layer that serve as the crucial diagnostic sign allowing to distinguish this epithelium from transitional epithelium when examining histological slides.
Topography: this type of epithelium lines the oral cavity, esophagus, conjunctiva, and forms the anterior corneal epithelium of the eye.
Features of the cornea and cuboidal epithelium
The anterior corneal epithelium has a unique histological feature. Beneath it lies the anterior limiting lamina—a specialized, very thick basement membrane. Unlike membranes in most other organs, it is clearly visualized under light microscopy as a wide oxyphilic band.
The body also contains stratified cuboidal non-keratinizing epithelium. This is a rather rare tissue type. Its main localizations are:
- Excretory ducts of salivary, sebaceous, and sweat glands;
- Columnar zone of the anal canal of the rectum;
- Shells of growing ovarian follicles.
Briefly: answers to topical questions
Which layers are distinguished in stratified squamous keratinizing epithelium (epidermis)?
In the cited sources, the following layers are listed for stratified squamous keratinizing epithelium:
- Basal layer.
- Prickle cell layer.
- Granular layer.
- Cornified layer.
The skin epidermis is represented by stratified squamous keratinizing epithelium. Such epithelium is also indicated for the gums and hard palate; for the upper surface of the tongue, stratified squamous partially keratinizing epithelium is described.
To memorize the three layers of stratified squamous non-keratinizing epithelium from bottom to top, use the phrase: "Basal → Prickle → Squamous".
Class and exam questions
Where are stem cells localized in stratified epithelia?
Stem (cambial) cells are located exclusively in the lowest — basal layer, which contacts the basement membrane.
How to distinguish stratified squamous non-keratinizing epithelium from transitional epithelium on a slide?
The main feature is the nuclei of the superficial layer. In stratified squamous epithelium, they are narrow, rod-shaped, and oriented strictly parallel to the surface of the layer.
How are the cells of the prickle cell layer held together?
Cells of this layer have short processes ("prickles") that adhere to each other via strong intercellular contacts—desmosomes.
What else to study on this topic
- Morphology of stratified squamous keratinizing epithelium
- Keratinization process and formation of cornified scales
- Transitional epithelium: mechanism of layer thickness change (urothelium)
- Molecular structure of desmosomes in the prickle cell layer
- Histophysiology of the anterior limiting lamina of the cornea
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